The present invention relates to the synthesis and purification of nanotubes and nanofibers. Nanotubes and nanofibers are important subclasses of nanostructured materials. Nanostructured materials are materials that have at least one dimension in the length scale of 1 to 100 nanometers. It is believed that nanostructured materials will enable the creation and use of structures, devices and systems that have novel properties and functions because of their small size. These novel and differentiating properties and functions are developed at a length scale of matter typically under 100 nm, and the length scale is maintained when the nanostructured materials are combined to form larger structures.
Nanofibers are nanostructured materials with a relatively high aspect ratio, that is, ratio of length to width. They have been made from both inorganic and organic materials, including ceramics, organic and biological polymers, metals, and semiconductors.
Nanofibers that are electrical conductors or semiconductors are also referred to as nanowires. They have been made from materials including metals, carbon, silicon, metal suicides, and other known semiconductors, for example, binary group III/V materials (GaAs, GaP, InAs, and InP), ternary III/N materials (GaAs/P, InAs/P), binary IINI compounds (ZnS, ZnSe, CdS, and CdSe), and binary SiGe alloys.
Nanotubes are structurally similar to nanofibers; however, they are hollow and exhibit a high level of molecular order. Carbon nanotubes, for example, appear to be made up of graphene sheets rolled to form seamless hollow tubes. Nanotubes may be electrical conductors or semiconductors. In addition to carbon, nanotubes have also been made from other materials, such as, for example, boron nitride.
Carbon nanotubes and single wall carbon nanotubes (SWNTs) in particular, have generated great interest in the scientific community. It has been reported that the electrical conductivity of carbon nanotubes is comparable to that of copper, that their thermal conductivity is comparable to that of diamond, and that their tensile strength is more than 100 times that of steel. It is predicted that the use of carbon nanotubes in composites can reduce weight by a factor of 5 to 10, while increasing the strength by a factor of 5 to 10 compared to a conventional carbon fiber matrix. No other material is known to be a superior field-emitter. It is therefore predicted that carbon nanotubes will improve many electronic devices, increasing their effectiveness and service life. For example, it is anticipated that the physical properties of carbon nanotubes will be of particular importance in the production of a new class of flat-panel displays used for computers, televisions, and other video screens.
It is apparent that, without facile techniques for synthesis and isolation, the technological utility of nanotubes and nanofibers, including carbon nanotubes, will not be realized to its fullest extent. In addition, the favorable physical properties that are predicted by theory are attenuated or lost when the nanotubes or nanofibers are contaminated by impurities.
Many of the proposed applications of SWNT, including nano-electronic devices, field emitters, gas sensors, high-strength composites, and hydrogen storage require reasonably pure SWNT materials.
Typical synthetic methods currently produce carbon tubes having a diameter in the range of 1–2 nm and arranged in the form of bundles. One particular problem associated with conventional synthetic techniques is that the intended SWNT is a minority constituent in the reaction product. Also present, for example, are amorphous sp2 carbons which coat the fiber walls and multi-shell carbon species which cover metal catalyst impurities that result from the catalytic production of conventional carbon fibers and SWNTs. It is a challenging problem to separate the desired SWNT from its accompanying mixture of amorphous carbon impurities, multi-shell carbon species and metal impurities without adversely damaging the carbon fiber or the tube walls.
Many purification procedures have been developed to remove the inherent contaminates from carbonaceous soots produced in an effort to obtain the desired SWNT. These methods include hydrothermal treatment, gaseous or catalytic oxidation, nitric acid reflux, peroxide reflux, cross flow filtration, and chromatography.
These treatments, however, tend to chemically destroy a significant portion of the desired carbon nanotubes, require excessive production times and, in the case of arc produced carbon nanofibers, have a marginal effect in purifying the desired carbon nanofibres from its impurities, such as amorphous carbon phases and graphitic carbon phases covering metal impurities. It is also unfortunate that the results of many of these purification processes have not been even semi-quantitatively determined with respect to the purity of the final product. Thus, they have been of little aid to the skilled artisan in advancing the understanding of purification procedures thereby reducing the predictability of successfully achieving a process of purifying SWNT in high yield and throughput.
Furthermore, most of the purification processes reported previously were for carbon-nanofibers produced by a pulse laser vaporization (PLV) process which inherently produces smaller amounts of catalyst residue and smaller amounts of multi-shell carbon phases as well.
One known method of removing residual catalyst particles and undesirable product phases is carried out under generally harsh conditions, such as, for example, heating at reflux in full strength aqueous HNO3 (67% by wt), or exposure to gaseous oxidizing agents at high temperature. Such methods, aside from their expense and inherent hazards, are deficient because the nanotube or nanofiber product is degraded under the severe conditions, causing a loss of product, or a decrease in the desirable properties of the product. When the residual catalyst particles are completely surrounded by shells or outer layers, the losses are especially severe, because it is difficult to distinguish between the nanotubes or nanofibers and the outer layers, which are both structurally and chemically similar to the desired product.
Another known purification method that has been used extensively for SWNTs is the so-called “selective oxidation” in air or oxygen of undesirable minority phases, such as amorphous carbon, multishell carbon, and carbon “onions.” In fact, the oxidation is not completely selective, because, inevitably, some SWNTs are also converted to CO and/or CO2. Conditions such as temperature, time, flow of dry air, etc., are chosen to create a combustion which preferentially removes amorphous carbon, and, to a lesser extent, other carbons. Conditions are also adjusted to minimize the loss of SWNTs from the sample. A significant portion of the SWNTs is usually lost in selective oxidations, nevertheless, because the SWNTs are structurally similar to the multishell carbon encasing the metal particles, and therefore also react with dry air at approximately the same temperature. Reported yields of the carbon nanotubes for different purification approaches vary from 1 wt % to up to 25–30 wt %.
Several recent publications report that heating the crude SWNT reaction product in dry air in the temperature range 300–500° C. was carried out as a first step whose purpose was to remove the amorphous carbon and to weaken the multishell carbon covering the metallic particles. This oxidation step, like the selective oxidation described above, cannot be carried out without a significant loss of SWNTs.
Thus, there remains a need for improved methods of isolating and purifying nanotubes and nanofibers, especially SWNTs.